Carbon Sequestration in Deep-Sea Sediments
8 August 2006
A team of researchers has concluded that deep-sea sediments could provide a virtually unlimited and permanent reservoir for carbon dioxide. The researchers estimate that seafloor sediments within US territory are vast enough to store the nation’s CO2emissions for thousands of years to come.
In a paper published in the Proceedings of the National Academy of Sciences (PNAS), the authors show that injecting CO2 into deep-sea sediments more than 3,000 meters water depth and a few hundred meters of sediment provides permanent geologic storage even with large geomechanical perturbations such as an earthquake.
The exciting thing about this paper is that we show that CO2 injected beneath the seafloor is sequestered permanently. CO2 injected underground on land is buoyant, and hence has the potential to escape back to the surface.
This is not the case under the deep ocean. Because the ocean floor is so cold, liquid CO2 stored beneath the floor is denser than water and will not rise to surface. Furthermore, the top of the injected CO2 plume will form a hydrate, an ice-like solid that plugs up the pore spaces, self-sealing the injected CO2 plume into the deep sea sediments.—Charles Harvey, MIT
Injecting carbon dioxide into seafloor sediments rather than squirting it directly into the ocean traps the gas, minimizing damage to marine life while ensuring that the gas will not eventually escape to the atmosphere via the mixing action of ocean currents.
At sufficiently extreme deep-sea temperatures and pressures, carbon dioxide moves beyond its liquid phase to form solid and immobile hydrate crystals, further boosting the system’s stability. The scientists say that thus stored, the gas would be secure enough to withstand even the most severe earthquakes or other geomechanical upheaval.
The scientists note that thin or permeable sediments are inappropriate for carbon dioxide storage, as are areas beneath steep deep-sea slopes, where landslides could free the gas. They add that further assessment of the mechanical feasibility of delivering carbon dioxide to the seafloor, as well as study of possible effects on sea levels, is needed.
Other researchers have proposed storing carbon dioxide in geologic formations such as natural gas fields, but terrestrial reservoirs run a risk of leakage.
In terms of capacity, about 22%, or 1.3 million square kilometers, of the seafloor within the US’ exclusive economic zone is more than 3,000 meters deep. The researchers calculate that the annual US emission of carbon dioxide could be stored in sediments beneath just 80 square kilometers.
Supplying the energy demanded by world economic growth without affecting the Earth’s climate is one of the most pressing technical and economic challenges of our time.
Since fossil fuels—particularly coal—are likely to remain the dominant energy source of the 21st century, stabilizing the concentration of atmospheric carbon dioxide will require permanent storage of enormous quantities of captured carbon dioxide safely away from the atmosphere.—Daniel Schrag, Harvard
The research was funded by the U.S. Department of Energy, the Merck Fund of the New York Community Trust and the Link Foundation.
“Permanent carbon dioxide storage in deep-sea sediments”; Kurt Zenz House, Daniel P. Schrag, Charles F. Harvey, and Klaus S. Lackner; Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0605318103
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